


Big Brother Blue (and maybe a little lewd, too!)

by DraconSinss



Series: What Comes After Thrice, Anyway? [1]
Category: Undertale (Video Game)
Genre: Alternate Universe - Underswap, Anal Sex, Baby's first fuckfic, Bara Blue, Bara Sans, Blow Jobs, Breeding Kink, Dirty Talk, Dom Sans, Dom blue, Fondling, Incest Kink, Knotting, M/M, Mild Humor, Post-Coital Cuddling, Praise Kink, Rough Sex, Sub Papyrus, Swapcest - Freeform, This wasnt supposed to be this lewd, Underswap Papyrus, Underswap Sans, Undertale Skeletons in Heat, Undyne does squat diddly shit so she doesn't get a character tag, minor food kink
Language: English
Status: Completed
Published: 2016-10-30
Updated: 2018-03-21
Packaged: 2018-08-27 20:23:41
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 2
Words: 7,353
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/8415448
Author URL: https://archiveofourown.org/users/DraconSinss/pseuds/DraconSinss
Summary: After years of being called tiny, Underswap Sans is finally gifted with an incredible height. It's too bad he's too busy trying to fuck his brother to enjoy it.





	1. Chapter 1

**Author's Note:**

> I swear to god this was supposed to be a cuddle fic until it wasn't.
> 
> Also there's a bit of non-conISH themes in the foreplay that I personally felt didn't need a tag. Papyrus tries to make a point on consent before anything super serious happens. But if anyone has a problem with it you can tell me in the comments and I can change it.

All his life Sans the skeleton was teased for being tiny. Though he still preferred the term ‘vertically challenged’. Sans loved his brother, he really did. (Although many residents of his home village would argue a bit too much). Except it annoyed him how his loving brother would treat him like the younger one, despite the fact that Sans had raised him. His younger brother had been taller than him nearly all his life and had even stolen the role of authority around the house. Of course everyone thought he was the older one, Sans mused, he was taller.

 

Even Captain Alphys of the royal guard had looked at the small skeleton and told him he would never have the chance to work with her in her force. He worked hard to get her to accept him as her student. It wasn't a lifetime opportunity he wanted to waste.

So, when Sans awoke nearly five hours after his normal scheduled time with Alphys, he simply didn't have time to feel the ache in his bones or magic.

 

He quickly got dressed, (had his clothes always been so small?) and zoomed past the living room into the kitchen to grab a light snack.

 

He yelped as he bumped into the counters multiple times and flinched when the refrigerator door slammed open. He grabbed some leftovers and placed them on the counter, of which seemed much lower than normal. Sans rolled his eyes, figuring Papyrus had something to do with everything being so small today.

 

Speaking of his younger brother, Sans noticed him staring from the doorframe of the kitchen. He seemed… smaller. Not to mention adorable! Is this how everyone seen him? Stars appeared in Sans’ eyes while he bounded towards his brother, leaving unheated tacos on the counter. Teeth bared into a smile, he loomed over his younger brother, taller than him for the first time since Papyrus was 8. Without hesitation, he scooped his brother up in a hug and carried him bridal style. “WOW, BROTHER! YOU'RE A THOUSAND TIMES LIGHTER NOW THAT YOU'RE SHORTER THAN ME!”

 

Papyrus blushed, face blooming into an orange color as he scrambled to get a grip on something in his shock. His phalanges clung tightly onto his brother's blue kerchief as he sputtered for a response. “sans?! bro, no offense but what happened?”

 

Sans didn't seem to hear the question as he buried his face into his brother's orange hoodie, taking odd intervals of breath. Papyrus’ eyes widened and he pushed his brother's skull away from himself. “bro, are you sniffing me?”

 

Sans nodded quickly, smiling. Under all that smoky, sweet and dirty smell was his brother, all right. 

 

With a click of his fingers and a prompt  _ woosh,  _ Papyrus teleported from the kitchen onto the main room couch. The other skeleton just trailed after him, picking him up like a doll and sitting down. Papyrus blushed as he was sat on his now big brother's lap. It was nothing they haven't done before, despite their switched roles. “sans, seriously. what happened to you?”

 

Sans sighed and pulled Papyrus closer against him. The now smaller skeleton shivered at the closeness. Warm. Safe. Small. It was pretty great. “I’m not sure, Papy. But you're so small it's so hard not to cuddle you like this.” His voice rumbled deeply.

 

The more Papyrus struggled, the tighter his older brother's grip became. So he did what he was best at and quit, going limp in his brother's giant arms before the gip became too tight. “we need to call Undyne”.

 

The mass embracing him purred. “But Papy! You're so small now, she might break you.” He inhaled and leaned down to nuzzle his brother's face.

 

Papyrus stiffened slightly, before relaxing. His face was nearly completely orange. “sorry sans, but I haven't changed a bit. I think you just…  _ grew. _ ”

 

Sans froze before leaning away from his brother and looking around the room. Everything was smaller, not just Papyrus… His eyes became stars. “Wowie! That makes much more sense. No wonder everything is so small.” 

Suddenly, he was back to leaning against his brother affectionately. “I still wouldn’t dream of having her slimy, putrid smelling fingers touch my Papy, though.”

 

Papyrus suddenly shook. “no, bro something’s wrong. we need to go to Undy- AH!”

 

Sans snarled beside his brother's head, eyes livid. He gripped Papyrus' hips and pulled them against his own. “STOP TALKING ABOUT HER.” He smiled sweetly at his brother's whimper as he grabbed his orange skull and angled it towards his.

 

“sans, you’re scaring me.” Papyrus tried to move his head with little success. His eyes closed as Sans leaned closer. His warm breath puffed onto his cheekbones, encouraging his blush. The bigger monster paused.

 

Sans’ eye lights shrunk into barely visible pin pricks as he let go of his brother. His baby brother, oh god… 

 

“I'M SO SORRY PAPY!” The skeleton explained, hugging his brother around the chest and squeezing him. He let go after hearing a pained noise. “I didn't mean to frighten you, I just don't think... I couldn't think. I'm so sorry.”

 

Papyrus took shaky breaths, trembling only slightly. “it's fine. I'm fine we just need to get you to see someone about this.” He gestured to his brother's big body.

 

Sans nodded enthusiastically. “I'll go make breakfast to eat before we go.” Papyrus went wide eyed when that grip was back on his chin. He closed his eyes when sans leaned forward with a blue glow emanating through his teeth. Something squishy and wet pressed against his cheekbones and left a shallow film of slick on his face, he shI've red slighty at the contact. He could hear his brother purr as he pulled away. When he opened his eyes sans was simply smiling down at him. “Tacos should be ready in ten, Papy.” The big monster walked out of his vision.

 

Papyrus could feel the warmth of his face as his knees shook. Damn, was this tempting. Except something was obviously wrong… there were just too many risks. He teleported out of the house and clicked on his phone. Undyne better damn well have an explanation.

 

“There's something wrong with sans.”

 

“Oh no! Is he okay?”

 

Papyrus leaned on one of the pointy trees in the snowy area. Not too far ahead of him laid his house, and inside that laid his brother. He watched the silhouette in the kitchen window worriedly. “not…. Really. He's huge, ‘dyne. At least a head taller than me. Not to mention width. He's at least two times bigger than me and I don't know what's wrong.”

 

“C-calm down. You said he’s bigger?”

 

“Yeah. He was also…  _ handling  _ me, sniffing me. This all just, heh, came out of the blue. I'm worried.” He suddenly felt shivers in all the places his brother touched him. It wasn't bad, let alone uncomfortable. Man, Sans was so warm. And soft. Like a pillow.

 

There were several noises of paper flipping before he could hear claws tapping against wood. “Has… Has Sans ever been in a heat before?”

 

Papyrus’ face flared orange. “How am I supposed to know that now?”

 

There was a sigh. “I-I don't know! But it would m-make your job a lot easier… and l-less awkward.”

 

Papyrus blushed further. “Undyne, you can't be serious.”

 

“I-I'm looking at all the documents we have on skeleton monsters, Pap. Normal heats he should be okay, but y-you need to know that this isn't.”

 

Papyrus let his eyes roam back to the silhouette of his brother standing in the kitchen. He could be done any second now.

 

“Papyrus, it's a dominant heat. And with how big I'm imagining him? It’s strong. He’s probably been suppressing it and needs to take care of it. So I would advise-”

 

“to let my gigantic brother fuck me?” To be honest, it didn't sound as off putting as it should have.

 

Undyne sputtered. “I w-was going to say get him some toys b-but that works too!”

 

Papyrus face palmed his glowing face. “sure…” He exhaled “thanks ‘dyne.” 

 

“N-no problem!”

 

He clicked off his phone and blinked back into the house.

 

Straight away he was assaulted from behind, hips being pulled back by one gloved hand and ribs being violated with the other. He gasped shakily as the hand began to stroke up and down his true ribs. “You left, Papy.” A hungry voice growled.

 

Papyrus whimpered when he felt something begin to poke his backside. Shit, already? How long had he been supressing this?

 

“I hope we don't need to make rules about leaving.” The deep voice said before it's hand curled under his rib cage and began to fondle his soul. Papyrus gasped, already panting with aroused magical beginning to pump through his system. 

 

“S-sans…” He sighed. He flinched slightly when he felt a magic lock on his soul.

 

The taller monster pulled Papyrus closer, continuing to gently poke into him with his clothed cock. Panting, Sans smiled. “It's so you don't try to run away from me, Papy.”

 

“I won't- AH!” Papyrus squeaked as his clavicle was gripped by Sans’ teeth, sharp canines digging shallow marks into his bone. The small prodding morphed into grinding as Sans licked the wound he created. Papyrus moaned gently as the hand stroking his chest gripped a false rib and began pumping it rhythmically. Each touch sending small spikes of pleasure to his mind.

 

“Good.” Sans breathed. “Papy I'm so hungry.” His movements slowed slightly. “I need you.” He moaned, pressing his cock harder into his brother's pelvic bone.”Please make me feel good, Papy. L-love you so much.” He grunted, pressing his hard-on onto his brother's bones.

 

Papyrus regained his breath and decided to cut in now before his brother's dominant instincts fully kicked in. Still breathing hard with orange dusting his features, he spoke. “yeah, bro. I love you too…” He gasped as his brother thrusted onto him through his clothes. “I can make you feel good. I can show you how to make both of us feel good. Would you like that? Huh, Sansy?”

 

Sans didn't stop as he nodded next to Papyrus' skull.

 

“stop for a second. I'll show you.” Papyrus spoke. Sans reluctantly stopped and was quickly led to his brother's bedroom where he was sat on the bed. The younger skeleton hissed worriedly at the lust filled stare his brother was giving him. “sans, can you hear me? Say ‘yeah’ if you can.”

 

“Yeah, Papy. I can hear you.” Sans purred, taking off his gloves.

 

Papyrus paid no mind and he gave himself a second to gather his thoughts. “sans, you’re in heat.”

 

“Am I?” Sans purred, dragging his tongue over his teeth.

 

Damn it, he wasn't listening, was he? “Yes and I'm going to… do things. Things that I want you to trust me with so I can make you… us… feel good. Okay, Sans?”

 

Sans just growled. “What are you waiting for?”

 

“For consent. Not that I think you'd mind at this point… but I just want you to know that I love you, bro. I care for you and mushy crap like that. Also that this type of thing is more… private than most activities and it means a lot to me that you give me a clear, focused go-ahead.” Papyrus knelt down on his knees in front of Sans, almost eye level with his hips. “Okay?”

 

Sans nodded, eyes clearing up and showing nothing but love and trust. “I love you too, Papy.” Sans groaned. “So, so much, Papy.”

 

Papyrus took a shaky breath. “okay.”

 

Sans moaned deeply as Papyrus began to grope his glowing blue bulge through his pants. He thrusted gently into the hold, groaning and hardening with each movement.

 

God, was he loud. Papyrus could feel his magic recollecting and pooling in his groin just from the noises his brother was making. “Am I making you feel good, Sans?”

 

Sans moaned and nodded. “Y-yes, oh god, Papy. I need more. Give me more, please.” Sans groaned, hips moving more firmly. He gasped as a wetness soaked through his pants. Lustful moans became louder as he struggled not to thrust at his brother's face. An orange tongue sucked gently at his pant fabric. Papyrus hummed against the fabric, only adding to the warm feeling Sans was collecting. The big monster' hips twitched foreward occasionally in want. He groaned and purred in pleasure until the stimulation was removed. He growled. “Why did you stop?”

 

Papyrus broke the weak string of magical spit between his mouth and his brother's crotch. “You don't want to take care of your little brother, Sansy?”

 

Papyrus stood off his bony knees and began to strip off his hoodie. Sans watched in silence, his face clearly displaying his train of thought before it settled on an expression that read a tinge of regret. Regret that he hadn't been being a good big brother, had he? “I can take care of you. Papy, I can. I want you to feel good, too.”

 

Papyrus slipped off his black sleeveless shirt and took off his slippers before going back into Sans’ arms. “Prove it. Prove to me you can take care of me, big brother.”

 

Sans growled and turned his brother so his stomach was on his back. He began licking an untouched part on Papyrus' clavicle. One of his now bare hands gently started pumping his sensitive spine and Papyrus gasped, back arching. Sans’ touches began sending sparks throughout his bones, heat rushing towards his arousal. “Sa- HAAAaaaannnns!” Papyrus let out a surprised moan as his brother's phalanges began to wrap around his magic cock.

 

Sans was licking and sucking the bone by Papyrus' head, moaning and purring as he pumped his sensitive spine, massaging the orange organ gently, and grinding his own length onto his brother's back. “Is this-” He groaned softly, magic still dripping from his mouth onto the younger’s clavicle. “Good for you? Am I taking good care of you, Papy?”

 

Papyrus whined, bucking into his brother's hand. “Almost, big brother.” Sans groaned at the title. “Need you. I want you to fuck me. Suck me, Sans. Put your dick in my ass, please.”

 

Sans growled as he suddenly flipped his brother face down, ass up on the bed. Papyrus gasped and breathed hard into the mattress. His hips were grabbed by two very big hands and pulled up to Sans’ face where he breathed on the concentrated magic. Soon enough, it formed a tight orange ass that the Dom wasted no time to start licking.

 

Papyrus shuddered and moaned as that cold long tongue dragged over his hole, sending electric arousal through his spine down to his head to land at his twitching needy cock. The magical organ was dripping with precum, beading at the top until it became too much and dripped down the length of his dick. The teasing only made his phalanges cling onto the bedsheets.

 

Sans panted and inhaled his brother's aroused scent. He growled and groaned before thrusting his tongue in deeper and lapping at the magical insides. Papyrus could only try to buck his hips backwards into the deep vibrations of his brother's groans, moaning and whining at the restraint Sans’ arms provided.

 

“Sans.” He gasped, causing the bigger to stop. Papyrus whined and bucked his lips slightly. “Sans, please, big brother. I want it. Fuck me, Sansy.”

 

Sans placed himself on top of his brother, his clothed front laying on the other’s naked back. His arousal slid between orange cheeks, length settling and rubbing gently against Papyrus’ hole. He leaned down to speak in his brother's neck. 

“Language, Papy.” He smirked before licking his brother's clavicle and biting down. One hand guided his giant conjured penis into his brother's ass. 

 

He groaned at the resistance and moaned at his brother's pained cries. Once he was fully in, he grasped his brother's cock with the hand that was just on his own arousal and began gently stroking. Not enough to get him to climax, but just enough to be a tease. His other hand crept along Papyrus' ribs attempting to sooth.

 

He let go of his bite and licked at the bone marrow that had started to leak through, feeding it healing magic. He purred at his brother's sobs and licked whatever tears were threatening to fall. He waited for the tears to stop running down his face before giving an experimental thrust.

 

When Papyrus moaned, something snapped and he growled, biting into the first shallow marking he had created earlier that day. His hips thrust hard and deep, rocking his thick cock into his brother's hole at a decent pace. The monster beneath him cried out and moaned loudly as they shook the bed. Hell, maybe even shook the house.

 

There were more laps at the markings. Papyrus was Sans’ now and that made the taller skeleton groan. His hips began thrusting faster, his brother's ass massaging his dick so perfectly. The pressure around him only made him crave more. “Mine.” He groaned, thrusting erratically. Papyrus' dick bobbed and bounced at the pace. A steady stream of precum beginning to leak out of him into a small pool beginning to form on the mattress. 

 

“MINE.” Sans yelled, dick swelling inside his brother's tight, tight entrance. He panted and groaned as he fucked his little brother hard into the mattress. “Gonna fill you up, Papy.” He breathed, groaning. “Gonna be my momma, brother-!” A roar ripped through his throat before he chomped down onto an unmarked spot on Papyrus’ clavicle.

 

Papyrus screamed and thrusted his hips towards Sans as his brother's seed was released spurt after spurt inside of him. His own cum shot through his length like a party canon, decorating the mattress in white ribbons.

 

Sans groaned and hugged his brother closely, pupils wavering into steaming blue hearts. Blue magic wisped away from him in visible waves before he closed his eyes and laid beside his mate, still attached and still sending spurts of cum into him. Papyrus’ eye sockets were closed and his harsh breathing was trying to calm. Sans purred and soothed his brother's skull. 

  
The bigger monster smiled with satisfaction before turning his brother's skull towards his own and clanking their teeth together gently. He rocked his hips slightly, trying to ride out his orgasm a bit more while also not waking his brother. He purred. “I love you, Papy.”

Papyrus groaned lightly. "Love ya."


	2. Aftercare + more smut whoops

**Summary for the Chapter:**

> I needed some non sin attached to this. So I added a fluffier aftercare chapter... that still has sin. Whoops.
> 
> (Edit: ^that first note was from over a year ago: this is 70% more fucking than i intended^)

**Notes for the Chapter:**

> All sanses are lazy pieces of shit at heart. No exceptions.
> 
> Also sorry for taking over a year to update this! My primal urges have been more honed im towards gore and shitposting rather than smut and lovening.
> 
> AND HOLY SHIT THANKS??? FOR THE KUDOS? LIKE DANG BROS I WISH I DESERVED IT MORE HA HA-
> 
> Also thank you for those who commented, expecially the more recent ones, this fic would have rotted if it weren't for you. <3

 

Sans groaned. His head was filled with warm cotton, pressing comfortably against the insides of his skull. It made his surroundings difficult to comprehend, yet he found the small distraction soothing. His face snuggled further into his fluffy bed sheets and his grip affirmed itself on whatever it was he was holding( -a pillow?). He hadn't bothered to open his eyes to check. It was warm, almost a head shorter than him and something whispered to his soul...  _ This was to be protected at all costs. _ He held it a bit tighter, purring in his sleepy state.

 

He hummed when the small mass in his arms stirred slightly. “...Sans?” He ignored his brother's voice and turned to his side, still holding onto the comforting object. “Bro, I know you're awake now. Mind letting me go?”

 

Oh, so  _ that's _ what he was holding. Sans groaned,a long drawn out sound. He buried his face into his brother's neck, uncharacteristically exhausted. “Don't wanna.” 

 

“bro… come on, I've been up for hours. I feel gross.” Papyrus whined. It wasn't a total lie. He had woken up earlier, but had given up on getting out of his older brother's grip and had simply fallen back to sleep. He had managed to reach his sweater and slip it mostly on without waking his brother. Until now, that is.

Sans only gave his brother a small squeeze. “later, babe.”

 

A grin spread across his face when he felt the smaller monster squirm slightly in protest. Papyrus huffed before going limp in his arms. His breathing steadied and Sans sighed.

 

They laid there for a few minutes before Papyrus blinked open one of his eyes, his gaze landing on the clock. Sans was going to be late again... oh well. He wiggled backwards, this time deeper in his brother's hold. He almost laughed at his next thought: Alphys busting through the door, guns blazing and eyes raging, ready to swoop Sans over her shoulder and whisk him away to training… only to see him and her student stuck here in bed. He'd bet no one could separate them if they tried.

 

A hand rubbed his shoulder blade. He heard Sans give him a gentle chuckle. “What are you laughing at, Papy?”

 

Papyrus hummed in thought. “You're going to get in trouble with Alphys.” He sang teasingly, flipping over so he could face his brother. His chin was tilted upwards by comforting hands.

 

_ Clank. _

 

Sans’ mouth rested against his. Calming waves of magic pulsed through him in miniature bursts. His soul warmed. After only a few long seconds, they parted. Sans gave him a smile, sharpened canines making themselves more pronounced. “Want me to get you cleaned up, mate?”

 

Papyrus felt his heart suddenly leap at that title. Damn, his bro was so cool. “Yeah, bro. Thanks.”

 

Sans purred and his grip tightened, firmly holding the smaller form, (Papyrus blushed slightly as a hand snuck under his pelvis in order to support the hold), he hefted himself up and off the bed. Papyrus tensed slightly as he was carried towards the bathroom, not able to remember clearly the last time his brother was able to lift him up this high. (Even with his normally small body, Sans could still carry him. Although, as could be imagined, it wasn't that much higher than, let alone barely the same size as, Papyrus’ own height.) It was nice.

 

The taller sat on the edge of the bath, turning on the tub. He stroked Papyrus’ head softly as the water ran. “Want it warm, Papy?”

 

Papyrus nodded. “Sure. I wouldn't want to be chilled down to the-”

 

Sans suddenly growled. “Don't you finish that, Papyrus.”

 

Papyrus only grinned. “Bone.” He laughed as his brother face palmed in mild frustration. That look on his face, the pure expression that made what he had just heard show perfectly through his malleable bone. Papyrus gasped in surprise as a big blue force pushed him backwards without warning. 

 

He thrashed to get his head out of the already rising water with his heavy clothes. Distantly, he could hear the faucet tap switching off. “bro, how could you?” He said impassively.

 

“Tch. It's warm water, Papy.” Sans assured, starting to lean forward. He grabbed the hem of the soaked skeleton's sweater and tugged it up and over his head. Gladly, the other complied and lifted his arms so the garment easily slipped off his frame. Orange began to paint Papyrus’ face. Sans smiled in return at the display of colors. “Want help bathing, Brother?”

 

He stared at his brother's questioning, but loving gaze. “I'm not going to wash myself, am I?”

 

Sans batted Papyrus lightly and smiled at the youngest’s laughter. “Of course you aren't, love. That's a job only someone capable of the level of caring I have could do, isn't it?”

 

Papyrus hummed as he slipped gradually lower into the water. Chin deep, he gave his brother a smile. “Only you, bro.”

 

Sans hummed back, twisting around to get a wash cloth before returning his undivided attention to his brother. “Only me, and only ever you, mate.”

 

The cloth began to scrub at the laziest brother's shoulder plates. He blushed. “Thanks bro. I didn't think you'd actually start bathing me.”

 

It was quiet, only the sound of water being poured on Papyrus’ back could be heard. They both became aware of their own breathing in the calming moment.  _ Vmmmmm _ . They could hear their own souls hum as they spread magic through their bodies. 

 

Papyrus exhaled as he let his thoughts run rampant in his mind. _You should both talk about this._ _He's still in heat. He's going to want to do it again. What if he wants children?_ Papyrus hummed. _Wouldn't be_ that _bad, right? Maybe they could start doing couple stuff after this. If Sans wants to, that is._ He tried shaking off his doubt. 

 

“This is nice.” He commented.

 

“Nothing less than nice for my-” Papyrus inhaled as he felt the cloth reach down to his sacrum. Sans grinned. “ _ baby _ brother.” The big skeleton purred, nuzzling into Papyrus’ shoulder. “Maybe I can join you?” His other hand reached out to tease the younger’s rib cage. “Put a lil’ baby in ya? Twins would be fun. You'd be a perfect mother. Right, Papy?”

 

Papyrus shivered. Maybe it was Sans’ heat getting to him, or maybe it was a genuine opinion, but he'd be damned if at this moment he said he didn't want that. He gasped in mock horror. “And get me all messy again?”

 

The cloth was forgotten and Sans began to tease the pooling magic of Papyrus’ crotch. Suddenly, water flooded over the top of the tub and Sans was filling the space between his brother's legs. “Oh, please. You've always been filthy, brother.” He growled.

 

Papyrus gasped. He smiled and hummed contentedly, the soft strokes soothing against his bones. “And something tells me that ain't gonna change anytime soon.”

 

“I hope it doesn't. Maybe I can start punishing you for always leaving all your stuff laying around.” Came the reply. Leaning in again, the larger monster clicked open his jaw. “You'd like that, right?” A large glimmering blue tongue made itself known and- ...and suddenly, Sans was interrupted by a bang on the door. Papyrus jumped, shoulder bumping into Sans’ skull. The bigger skeleton sighed with a growl, the pain to his head going unnoticed.

 

There was a few seconds of silence where Sans listened closely for any more interruptions. Almost a minute later, he huffed, satisfied that the disruption disappeared. He leaned forward, dragging his tongue along his brother's neck. He pecked the smaller on the cheek and smiled when he heard the adorable giggling “nyeh”s burst out from his brother’s-

  
  
  
  


_ BANG BANG BANG _

 

The knocking returned, wilder than before. “SAAAAAAAANS!”

 

Papyrus (ever amused) watched Sans growl in annoyance before his brother stomped away, not bothering to change his wet clothing or grab anything to dry with. Water dripped off of the bigger brother and seemed to soak into the carpet outside the bathroom. Papyrus sunk into the bath, disappointed. He watched the water spread across the nearby tile floor. Slowly his eyes closed and he whimpered. 

 

Fuckin’ cockblock.

  
  
  
  


The door was thrown open by an annoyed skeleton. He nearly filled the whole frame of the entrance as he towered over the friend that stood at the doorway. She stood strongly and held a mace (as well as an air of confidence) above her shoulders.

 

Through grit teeth, Sans spoke. “Wowie. How nice it is to see you, Captain.”

 

“HOLY FUCK.” The small yellow monster shrieked, ignoring her friend's curt tone. Her posture only faltered briefly, her claws digging into the wood of her weapon and her scaled tail briefly pointing upwards. She quickly made sure to straighten herself out.

 

“Alphys! Seriously, language.” Sans scolded, exuding a ‘I‘m not mad just disappointed’ feel.

 

Alphys couldn't take her eyes off her friend's enlarged height. “YOU'RE HUGE!”

 

A smile forced itself upon the skeleton's expression. He could feel his temper was off and he just  _ really fucking missed his brother. It's hardly been five minutes, Sansy, keep it together. _ “You don't say!” He shouted.

 

“WHAT? Have you SERIOUSLY not noticed?! There's no way.” Alphys dropped her weapon on the snow. “Anyways, WHERE HAVE YOU BEEN?! YOU NEVER EVER MISS TRAINING and… ugh, I was worried. BUT THAT'S BESIDE THE POINT!”

 

“Well, Captain-”

 

“AND WHY WEREN'T YOU AT YOUR POST?” The small monster suddenly sighed, head falling into her hand. She shrugged before looking up at him in a serious, yet joking, squint. “And I thought you were serious about being a guard.”

 

Sans gaped, shocked. He took an angry breath before growling.  _ How DARE she insinuate that I don't want to be in the guard. _ “I AM SERIOUS! I WAS JUST…” The skeleton grimaced, searching for a good excuse. Anything was better than  _ oh hey I was just fucking my brother all day yesterday before passing out.  _ “I WAS BUSY!!!”

 

Alphys scoffed. “So YOU think just because you're TALL now, that YOU can prance around and do what you want? Nuh uh, Sans. IF YOU WANT TO EVEN BE CONSIDERED AS GUARD, YOU HAVE TO WORK YOUR ASS OFF, TALL OR NOT!”

 

“but-” Sans spared a quick look back inside the house. Standing there on top of the stairs like a dream was Papyrus, orange blush and all, wrapped in the softest dark green towel they had. He was staring at Sans with what must have been a gentle, submissive, hopeful lust. It would have seemed closer to innocent if Sans hadn't known better. His poor baby brother must have been shaking with need for Sans to-

 

“NO BUTS! TRAINING IS NOW, AND UNLESS PAPYRUS IS DYING IN THERE, IF YOU'RE LATE YOU'RE  _ OUT _ !” Alphys shouted impatiently (And irrationally, she'll realise later). She observed Sans with a harsh glaring stare that was overlaid with worry.

 

There was a pause. The silence almost choked out the air with how saturated it was.

 

Sans snarled at the choices laid out before him. So what? She was making him choose now? Not that she knew the circumstances, it was still Alphys up against his brother. With one long look inside the house, at his beautiful brother, Sans made his decision. “You're right, Alphys.” He sighed, voice down.

 

He locked his eyes with hers, the brilliant blue of his eye lights shining with determination, passion, and something the commander couldn't place. Certainty. He took a sharp inhale of breath before revealing his answer.

  
  
  
  
  
  
  
  
  


“Papyrus is dying.” He slammed the door in her face and locked it, also using a makeshift magic lock just in case.

 

Papyrus himself watched with wide eye sockets, his blush thickening across the off-white of his bones. “uh, sans?”

 

“Bed. Now. Before she finds a way in.” Sans moved into the kitchen, leaving a startled (yet extremely touched) Papyrus to head into the bedroom.

 

Seconds later, Sans joined him. Food items of many different varieties were held in his arms and he plopped them onto the ground, keeping hold of a select few. 

 

He walked straight towards Papyrus, who was lounged and nude on his bed (bless his little brother), with only one goal in mind.

 

“Drink.” He commanded. A bottle of water was pushed between Papyrus’ teeth. It took a second or two of orientation but the lanky skeleton was able to start gulping it down. His brother looked at him dreamily, happy to provide care for his mate. However rushed it may be.

 

The empty blue-tinted water bottle was soon replaced with a soft blueberry muffin. A suitable snack, in Sans’ opinion.

 

As Papyrus took another bite from his muffin, he couldn't help but notice a strong heated hand carefully traveling up his leg. “Mmmph?” He questioned, the sound muffled by the delicious treat.

 

“Before she breaks in, papy.” Sans said as if it were the most simplest thing in the world. He leaned forward before giving his smaller brother a kiss.

 

Papyrus mewled, letting his tongue squirm its way into his brother's-  _ His big brother's mouth. _ He couldn't help release a small moan at the thought. The chunk of muffin was stolen from behind his teeth.

 

The bed dipped. Sans nestled his hips close between Papyrus’ and swallowed the stolen treat. The bath water still soaked in the eldest brother's trousers dampened the union. He could feel Sans’ smirk as he began grinding his hips against Papyrus’ unclothed pelvis. The smaller of the two shivered at the cooled fabric against his bare bone.“It's only been hours, but  _ fuck _ , Papyrus. I've missed you more than my giant cock missed your tight,  _ tight _ , little entrance.”

 

The younger brother almost squeaked in embarrassment at the bigger’s insistence. “i-it's not that sm- ahn~ sans, fuck.” 

 

Papyrus’ speech was cut off when a large hand began to gently fondle his sensitive sacrum. Small circles stroked against his sacral foramina, and with every caress he couldn't help but have a high-pitched squeal of arousal ripped from his breath. 

 

There was a banging outside, but with Sans covering his senses it seemed much too distant for him to care. “Come on, Papy. F-form that tight ass I can p-pound into. You're s-so  _ good  _ at it.” Sans leaned into Papyrus’ sore clavicle and gently kissed the resting bitemarks and bruises. He spoke with a dreamy honesty layered in the sweetness of his breath. “Always perfect.”

 

Papyrus could feel the heat only rise further onto his skull as his magic began to pool in his pelvic inlet. His big brother smirked before he spoke. “My perfect little boy.” 

 

The smaller skeleton inhaled and almost lurched at how quick his magic formed. An orange entrance, puckered and tight, as well as a well-endowed cock slick with precum. “s-sans…” Papyrus couldn't help but look away in embarrassment. He'd formed so quickly-

 

“It's true!” Sans grabbed his brother's chin and directed his attention on himself. “You're so perfect brother.” He declared, quietly in an almost-whisper. He thrust his hips down, onto his brother's unclothed length. Papyrus nearly choked and Sans smiled. “L-let me show you.”

 

The banging noise got louder, almost breaking through their veil of arousal. Sans thrust his hips again. “You’ve always been so handsome.” He grunted, maintaining a steady grind of his hips against his brother's cock. His hands slid down the younger brother's face onto his chest, and he slurped up the bit of drool that was about to leak off his chin.

 

“Pappy you're so smart~!” He crooned, hands practically ripping off his own pants.

 

He slid his brilliant blue cock next to his brothers’ and couldn't help but whimper. What happened next was bliss. He rolled his hips and at the contact Papyrus let loose a surprised high pitched moan that sounded like a squeaked out “s-sans!”.

 

“Mmm, Pappy! Your cock is so wet, ah.” Blue moaned, letting his hips thrust against his little brother's length. He almost giggled at the size difference, at this heat size, his dick was only a little bit longer than his brother's but twice as thick. He couldn't help but lick his lips. “And so much smaller.”

 

Papyrus flushed, his face drowning in his orange magic. “B-bro…” He flustered, unable to think of what to say. He wasn't even-! When Blue thrusted his hips again, all the younger brother could do was moan. “Em- embar-AH-s-sing…” He found himself moaning out.

 

Alphys’ shouts steadily became louder, but with a wave of his hand and a bit of focus (not to mention the incredible amounts of pure  _ will) _ the room door was locked and soundproofed by Sans.

 

Sans suddenly stopped thrusting his hips and his little brother whined. The bigger monster giggled.”Calm down, mate…” Sans drifted his focus down, slowly lowering his much bigger body until he was faced with the bottom of his brother's pelvis. He nibbled Papyrus’ femur. 

 

“Want a treat, Papy?” The skeleton slowly opened up his mouth and stuck out his tongue, saliva stringing his teeth and tongue together. He dragged the ecto muscle up… and up.

 

Papyrus could only blush as he quietly responded _. _ “Y-yeah bro.”

 

Blue magic slowly swirled around Sans’s vertebrae. It curled until a thick piece of ecto flesh was formed. Sans’s smile curled into a pleased grin. A throat. It moved, expanding with every breath Sans took and pulsating whenever he swallowed.

 

The skeleton chuckled, the baritone sound barely registering with the very intrigued Papyrus. “Maybe this is more of a treat for me, huh?”

 

Sans ducked his head down and swallowed around the tip of his brother’s brilliant orange cock.

 

The corners of his mouth rose with mirth.

 

Papyrus hissed softly and whimpered.

 

Throat convulsing, Sans sank his head down closer and closer to his brother's hips. He couldn't help his deep-vibrating moans once the noises streaming from Papyrus’ mouth rose in volume and intensity.

 

He pulled away from the length with a distinct “pop” as his hands went to work, stroking at the base of his brother's penis.

 

“Good so far, brother?” He gave the hole at the top of Papyrus’ dick a small prod with his tongue. “Do you love it when your big brother sucks off your perfect cock?” His mouth wrapped around just past the corona and his eyes looked up to his brother. Big blue beautiful pools of endless lust and endless love. 

 

_ Papyrus could drown in them. _

 

“Bro,  _ please _ .” He begged, hips unable to thrust upwards due to his brother's heavy hands. “It feels good,  _ it feels good! _ Brother ple- _ hah… hah… hAH…” _ Moans poured from the leaner skeleton’s jaw. Then suddenly… everything stopped. Confused, he opened his eyes (when had he closed them?) and looked down.

 

His head bowed and sweat dripping down his skull, Sans was panting in a strenuous way. Worried, Papyrus leaned forward. “Sans…?”

 

The bigger skeleton waved him off, and Papyrus noticed the throat desummon. “I'm good, Papy. I just…”

 

Papyrus cried out. One second Sans’s head was facing downwards, the next that blue tongue of his was lapping at his conjured entrance.

 

Sans himself couldn't help but to smile. The way his brother became so concerned… the way he  _ begged _ \- moving his hips to rock against his face, the way he moaned-  _ loudly _ , the way he smelled, the way his puckered anus relaxed after the first couple of seconds tasting it, the way his  _ gorgeous  _ cock twitched deliciously without even being  _ touched!  _

 

Papyrus himself was having a hell of a time. He panted, the whole of his body limp- the entirety of his lower regions being an exception. As he let moans pour out from his mouth, his hips squirmed unreasonably.

 

The few times the lanky skeleton attempted to rub his own cock, his hand was shooed away by a very insistent Sans. He sobbed, already close but so far. He just needed a bit- “ _ more. _ ” He gasped out.

 

There were a few dull knocks against the bedroom door that the skeletons disregarded.

 

He gasped sharply and choked on a scream. A stinging sensation pierced his left ass cheek in tandem with an arousing pulse to his dick. His knees wobbled and he keened, pre dribbling out from his aroused length.

 

Sans let his teeth sink further into his brother's supple ecto flesh, testing the limits. It turned out the limit wasn't very far. Little droplets of magic seeped into his mouth and he lapped it all away. “You taste  _ so good. _ ”

 

Oh my god. Papyrus covered the majority if his face with one hand. This shouldn't be as arousing as it was. The pain. The white hot flash of pain. The sensation of his magic rushing through his body like adrenaline.

 

Sans chuckled at his brother's bashful demeanor.  _ Though _ , he reasoned,  _ now was a better time than any to take a step forward. _ He pulled his brother's hips against his own in one big motion. The other skeleton squeaked in response. “I'm going to fuck you, papy~” he crooned. “You're going to be so full of my cum I'll have to be extra careful, lest you burst!”

 

Unsettled, Papyrus opened his mouth, ready to question his brother. “Bro, wh- AHt…” Sans rammed his dripping cock into his brother.  _ Will i ever get another word in? _

 

“You're going to feel so g-good!” Sans started at a brutal pace, slamming over and over into his brother. Papyrus cried and scrabbled for purchase. He sobbed out broken moans. It hurt, his brother was going too hard too fast but… He inhaled sharply.  _ He loved it. _ He loved everything. His brother's husky moans, his scent, his size, his  _ power.  _

 

“Yes” he cried, catching whatever breath he could. “Fuck,  _ sans!”  _ He needed it. He needed this. 

 

“Fat with my cum, full of our children…” Sans basically growled, his fingertips digging into his brother's hips like claws. “Full of my c-cock.”

 

Papyrus grasped at his own length, clinging desperately in want for Sans’ cum. For his brother to fill him up completely.

 

As sans’ hips pistoned into him. Papyrus let the moans tear and pour out from behind his teeth. Every thrust went deeper and deeper inside him. Every thrust send another pulse to his already throbbing dick. He was so close he just wanted to cum he just  _ needed _ to cum. This AND earlier was too much. He was going to lose it.

 

“Sans sa-ah~ns please, f-fucking touch me. Just fucking touch me.” Desperate tears started to leak down his skull, debauched expression tantalizing.

 

Sans bit into his brother's shoulder and redoubled his efforts. “Do you want to have a baby?” He asked, the question muffled by the bone snagged between his teeth.

 

Papyrus moaned loudly, rocking his hips back. The still tender ecto flesh on his bottom was whipped hard with an ivory hand. There was a loud  _ whack _ that he'd be dreaming of for weeks to come. 

 

Sans growled. “Answer me!” As those perfect phalanges dug into the younger’s skin Papyrus screamed. “Y-yes!”

 

“Want my cum, brother?” Sans growled out.

 

The other panted, voice breathless and worn. Yet it held a desperation only sans would ever hear from him. “Yes yes!”

 

“Then take it.” Sans gripped Papyrus’ cock a bit too harshly as he came into his brother. He pulled papyrus even closer, slamming a hard swollen ball of concentrated dick into the sphincter. Papyrus came with a loud whimpering cry. Sans knotted him, his cock spurting in an obscene amount of cum. 

 

Papyrus felt his magic start to stretch in order to accommodate the flow of semen. It burned, as if he were stretching a stiff joint just a bit too far. It was pleasant. “F-fuck bro…”

 

Sans put a hand to the mess of cum laying on his brother's stomach. He held the substance to his mouth and allowed himself a taste of the taboo seed. Ambrosia.

 

His brother whimpered as Sans’ dick kept twitching. Kept filling him up. Sans thrust a bit, knot catching onto the flesh, drawing a few more soft overstimulated moans from Papyrus.

 

It was quiet. Sans pat his brother's distended stomach. Swollen and filled with his essence. “Perfect papy” Sans praised softly, rubbing the lump delicately. “...perfect. Love you.” He whispered.

 

Sleepily, nay, lazily, Papyrus turned his head to focus on his brother. “Love you too, bro.” He slid his hand over his brother's, enjoying the belly rubs. After all this was over maybe they'd do it again, without the intense heat fogging either of their vision.

 

_ Boom. _

 

Papyrus was shocked out of his daze in an instant. The door was suddenly kicked down. “SANS YOU- ...oh.”

 

Sans screeched. “OUT!!!! ALPHYS, GET _ OUT _ !”

 

“THIS IS WHAT YOU MEANT WHEN YOU SAID YOUR BROTHER WAS DYING?!?!” The yellow monster gaped, jaw dropped and eyes wide in mirthful surprise. “JUST TELL ME NEXT TIME YOU'RE IN HEAT, JESUS CHRIST I DIDN'T EVEN KNOW SKELETONS  _ HAD  _ HEATS.” She waved her arms around as she screamed, unable to contain her confusion.

 

“Alphys I'm knotted to my brother plEASE. JUST. LEAVE!!!!” Sans screamed, louder than before.

 

The dinosaur raised her arms defensively. “Fine, fine. Wait did you say knotted-” 

 

With a wave of blue magic she was flung out of the room.

 

Sans sighed and held his brother close. “Sorry papy-” he paused, Papyrus asleep. “Sigh…” he held his little brother close and buried his face in his bones, exhausted and ready to sleep.

  
He didn't even notice that by the end of it all, he had used up his excess magic. His achy bones were their original size. He was once again the smaller of the two brothers.

**Notes for the Chapter:**

> I hope it was enjoyable and... I... Got... A tumblr??? 
> 
> afreshpairofbones.tumblr.com 
> 
> \- mostly shitposts or cute animals lol. It's not the main url and im not sure how that works on tumblr but- I put a welcome message for y'all for when i was done this fic buuut... I wanted to edit this and make this a bit more readable??? Ehh??? Words??

**Author's Note:**

> I'm sorry. Idfk us!paps and this was overall horrible. :,)


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